The present invention relates generally to pendulum clocks and more particularly to clocks having a swinging pendulum which serves an aesthetic purpose but does not control the clock mechanism.
Some conventional weight driven or spring driven clocks utilize a pendulum to control the clock mechanism. Other clocks, such as electrically powered clocks, do not, and this is particularly true for mass produced, low cost clocks. However, many prospective clock purchasers consider clocks with pendulums to be desirable, from an aesthetic standpoint. To supply these tastes, clock manufacturers produce an electrically driven clock having an ornamental swinging pendulum, which simulates, in appearance, the pendulum in a pendulum-controlled mechanically driven clock, but performs no clock-controlling function.
One conventional manner of swinging or oscillating the ornamental pendulum in such a clock is to drive the pendulum directly with a cam mechanism driven by the same electric motor as drives the clock mechanism. Although this is satisfactory for short pendulums (e.g., up to about 6 inches in length), it is unsatisfactory for longer pendulums for the reasons explained below. For a given length and mass, a pendulum has a constant, unchanging period of oscillation (its "natural period"), which is the time it takes for the pendulum to move through one complete arc of its swing. The period is directly proportional to the pendulum length, i.e., the distance between the pivotal axis of the pendulum and its center of mass. A conventional pendulum consists of a very thin rod with a massive bob at the end of the rod so as to locate the center of mass at the bob.
When a pendulum is directly driven by a cam mechanism, or the like, there is imparted to the pendulum an oscillation period corresponding to the speed of operation of the cam. This oscillation period usually differs from the natural period of the pendulum, and the pendulum tends to resist the artificial oscillation period thereby resulting in jerky, uneven movements by the pendulum. Generally, for ornamental pendulums, the artificial oscillation period is longer than the natural period of the ornamental pendulum, per se.
A prior art solution to this problem involved two atlernatives. One alternative was to carefully control the length and other parameters of the pendulum to provide it with a natural period corresponding to the oscillation period imparted by the cam mechanism. The other prior art alternative, especially where the aesthetics or other considerations did not permit increasing the length of the ornamental pendulum, was to mount the ornamental pendulum in free swinging relation on a shaft oscillated by the cam mechanism and fix the ornamental pendulum to a heavy, non-decorative arm which extended above the pivotal axis of the pendulum and was hidden from view. The counterbalancing weight of the upwardly extending arm had the same effect as increasing the length of the pendulum.
However, in the case of the free swinging pendulum with the upwardly extending arm, unless the pendulum had a natural period identical to the oscillation period imparted by the cam mechanism, the pendulum would eventually stop swinging. To assure that the pendulum with the upwardly extending arm had a period identical to the oscillation period imparted by the cam mechanism, required precise dimensional tolerances and careful manufacturing practices with respect to both the cam mechanism and the pendulum; and these factors were inconsistent with mass producing low cost clocks.